Sometime in the 900's, three new inventions showed up and spread rapidly. They changed everyone's lives because they changed how people grew food.
The first of these was the plow. Prior to the invention of the plow people went into their fields and poked holes in the ground with a stick and dropped seeds into them. The plow was a way to turn over the ground in straight lines. People could use the plow on a field, then walk down the rows dropping their seeds. This was so much faster and easier then doing them one at a time. The amount of food being grown went up immensely.
The second of these was the horse collar. A horse can pull almost as much weight as an ox, but only if hooked up correctly. Horses were cheaper then oxen so more people could afford them. However to pull weight you can't just throw a rope around a horses neck; you would choke it. So, the farmers needed something to protect the horse's neck, and some very smart medieval person figured out how to make a horse collar.
The final important invention was the horseshoe. A horseshoe protects a horse's feet so that it can do the work you need without getting hurt.
With the amount of extra food being grown there was enough food left over that the noble was able to provide food for men who didn't have to work the field, but could instead learn how to fight better. This was the birth of the Knight.